Electronic security systems are known for detecting the unauthorized removal of articles from an area under detection. Such systems have been employed especially for use in retail stores to prevent the theft of articles from the store, and in libraries to prevent the theft of books. Such electronic security systems generally include an electromagnetic field which is provided in a controlled area through which articles must pass in leaving the protected premises. A resonant tag circuit is attached to the items, and the presence of the tag circuit in the controlled area is sensed by a receiving system to denote the unauthorized removal of the article. The tag circuit is removed by authorized personnel from an article properly leaving the premises to permit passage of the article through the controlled area without alarm activation.
Systems are also known for the electronic deactivation of a resonant circuit such that the deactivation circuit can remain on the article properly leaving the premises. One such system is shown in U.S. Pat. No. 3,624,631, in which a fusible link is in series with an inductor and burned out by means of a high powered radio frequency transmitter. The resonant circuit is interrogated by a swept radio frequency, the presence of this circuit in the controlled area causing energy absorption at the resonant frequency which is detected by a receiver for subsequent alarm actuation. Upon application of a swept frequency of higher energy than that employed for detection, the fusible link of the resonant circuit can be destroyed to deactivate the tuned circuit such that no detection is possible. Deactivation must be accomplished by a swept frequency transmitter operating at sufficiently low radiation levels to meet the requirements of the Federal Communications Commission, and thus, the fusible link must be extremely small and made of a material to allow fusing at low power levels. The small fusible link has a high resistance which appears in series with the inductor of the resonant circuit. This series resistance reduces the Q of the resonant circuit and thus reduces the sensitivity of the circuit to be detected. The current level at which the fusible link melts is determined by the geometry of the link as well as the heat conduction properties of the materials surrounding the fusible link. Thus, the fusing current is greatly affected by the material which cover and support the fusible link.
Another electronic security system is shown in U.S. Pat. No. 3,810,147 of the same inventor as the present invention, in which a resonant circuit is employed having two distinct frequencies, one for detection and one for deactivation. A small fusible link is employed in the deactivation circuit which also includes a second capacitor to provide the distinct deactivation resonant frequency.
The resonant circuit can have a resonant frequency which will vary within a range due to manufacturing tolerances. The deactivation frequency is at a fixed frequency, and thus the resonant circuit may not be tuned exactly to the fixed deactivation frequency. The series impedance of the inductor and capacitor at the intended deactivation frequency must be as small as possible in order to permit the maximum current to flow through the fusible link to cause burnout of the link. Therefore, the capacitor should have a value as large as possible, and the inductor, a value as small as possible. In actual construction, the inductor is formed as a single turn, and the capacitor is formed of plates as large as possible consistent with the economic and physical limitations of the particular tag circuit. The size of the capacitor increases the cost and size of the overall resonant circuit.